Charging a lithium-polymer (LiPo) battery to 100% in a smartphone is generally safe, but it can have some long-term effects on the battery’s health. Here’s a scientific explanation:

1. Battery Chemistry:

• LiPo batteries, like all lithium-ion based batteries, have an optimal charge range where they perform best and experience the least amount of stress. This range is typically between 20% and 80% of their full charge.
• Charging a LiPo battery to 100% regularly exposes the battery to higher voltage for extended periods, which can accelerate chemical reactions within the battery and lead to degradation over time.

2. Voltage Stress:

• When a LiPo battery is fully charged, the voltage per cell is around 4.2V, compared to around 3.7V when it is at 50% charge. Keeping the battery at or near this higher voltage level for long periods (like when the phone is left on the charger overnight) can increase the rate of chemical wear inside the battery.
• Over time, this can lead to reduced capacity, meaning the battery won’t hold as much charge as it originally did, and it may require more frequent recharging.

3. Battery Life and Cycle Count:

• A battery’s lifespan is often measured in charge cycles (one full discharge and recharge). Partial cycles (e.g., charging from 30% to 80%) are less stressful on the battery compared to full cycles (charging from 0% to 100%).
• Charging to 100% doesn’t immediately harm the battery, but consistently charging to full capacity can shorten the overall number of cycles the battery can endure before its capacity significantly diminishes.

4. Heat and Battery Degradation:

• Charging a LiPo battery generates heat, and fully charging a battery can exacerbate this. High temperatures can further accelerate the degradation process, especially if the phone is used intensively while charging (e.g., gaming, using GPS).
• Heat is a significant factor in battery aging, and maintaining a battery at 100% while it is warm can contribute to long-term damage.

5. Modern Battery Management Systems:

• Many modern smartphones include intelligent charging systems that help mitigate some of these issues. For instance, they might slow down charging as the battery approaches 100% or even delay charging to 100% until just before you typically unplug the device (a feature seen in some phones to optimize overnight charging).

Conclusion:

Charging your smartphone’s LiPo battery to 100% is not inherently bad, but regularly doing so can contribute to faster battery wear over time. For optimal battery longevity, it is better to keep the charge level between 20% and 80% and avoid leaving the battery at 100% for extended periods, especially in warm conditions. However, modern smartphones often include features to help manage these effects, so charging to 100% occasionally or as needed is generally fine.